Continuously cast slabs for grain oriented electrical steel sheet and method for producing said steel sheet

ABSTRACT

A continuously cast slab for producing grain oriented electrical steel sheets, comprising 0.025 - 0.060% of carbon, not less than 0.030% of manganese, not less than 0.010% of sulfur, 2.0 - 4.0% of silicon, not more than 0.005% of sol.aluminum, with the balance being iron and unavoidable impurities, and the ratio of SiO.sub.2 /Al.sub.2 O.sub.3 being not more than 1.1.

Matsuoka et a1.

Assignee: Nippon Steel Corporation, Tokyo,

Japan Filed: Nov. 29, 1972 Appl. No.: 310,261

Foreign Application Priority Data Dec. 3, 1971 Japan 46-97704 U.S. C1 148/110; 75/123 L; 148/3155;

148/111; 148/112 Int. Cl. H01f 1/04 Apr. 8, 1975 [58] Field of Search 75/123 L; 148/3155, 111, 148/110, 112, 113

[56] References Cited UNITED STATES PATENTS 3.671.337 6/1972 Kumai et a1. 148/111 Primary Examiner-Walter R. Satterfield Attorney, Agent, or FirmToren, McGeady and Stanger [57] ABSTRACT A continuously cast slab for producing grain oriented electrical steel sheets, comprising 0.025 0.060% of carbon, not less than 0.030% of manganese, not less than 0.010% of sulfur, 2.0 4.0% of silicon, not more than 0.005% of sol.aluminum, with the balance being iron and unavoidable impurities, and the ratio of SiO. /Al O being not more than 1.1.

2 Claims, 5 Drawing Figures PATENTEDAPR 8 I975 SHEET 1 (IF 3 FIG.I

FIG.2

Pmiminmmn saw 3 pg 3 FIG.

Multifier c1 CONTINUOUSLY CAST SLABS FOR GRAIN ORIENTED ELECTRICAL STEEL SHEET AND METHOD FOR PRODUCING SAID STEEL SHEET The present invention relates to production of a grain oriented electrical steel sheet having good magnetic properties from a continuously cast slab having a specially adjusted composition.

A silicon steel is composed of grains having bodycentered cubic lattice, and it is well known that the three l axes in the mutually perpendicular edge direction can be easily magnetized. Also, in a grain oriented electrical steel sheet, the easily magnetizable direction of the l00 axis is arranged in parallel to the rolling direction of the steel sheet, and the (110) plane is arranged in parallel to the rolled surface. ln crystallography this is designated as (110) [001] orientation by the Miller indices.

As above, in a grain oriented electrical steel sheet, the composing grains have a specific selected orientation, and the formation of such grains is attained by the so-called secondary recrystallization caused by selective growth of the primary recrystallization grains having the (110) [001] orientation through the final annealing of the cold rolled steel sheet of final thickness.

The grain oriented electric steel sheet, as a soft magnetic material, is used chiefly for transformers and iron cores of generators, and must have good magnetization characteristics (relationship between the magnetic field strength and the magnetic flux density) and iron loss characteristics (relationship between the magnetic flux density and the iron loss) as the magnetic properties.

The magnetization characteristics depend on the degree of the magnetic flux density (generally expressed by B value-wb/m caused within the iron core with respect to a given magnetic field.

lron cores having a high magnetic flux density (B can be obtained by a grain oriented magnetic steel sheet containing grains having a l00 axis along the rolling direction in a high proportion.

lron loss (generally represented by the value of W /50-W/kg-) is the energy loss consumed as heat energy of the iron core in a certain alternating magnetic field.

It is known that thedegree of iron loss is influenced by the plate thickness of the grain oriented magnetic steel sheet which composes the iron core, the amount of impurities, the specific resistance, the residual stress, etc., and it is also known that the influence of the magnetization characteristics is predominant.

Therefore, improvement of the magnetization characteristics (B of a grain oriented electric steel sheet is not only effective for reducing iron loss, but also able to minimize the size of electric appliances.

In recent years, industrialization of the continuous casting of steel has been energetically advanced, and the production of steel by the ordinary ingot-making method has been replaced by steel production using continuous casting.

Advantages of continuous casting, as well known, include shortening of the production day thanks to the shortening of the production process, improved productivity by savings of labour, and technical advantages, such as, uniform chemical composition among individual slabs by uniformity of chemical composition in the casting direction.

Therefore, also in the production of grain oriented electrical steel sheet, the adoption of continuous casting is expected to give technical and economical advantages, such as, reduction of variation in the magnetic properties, stability in the quality of silicon steels, and an increased production yield.

However, before adopting continuous casting for the production of grain oriented electrical steel sheet, it is necessary to solve technical problems due to the elimination of the steps of rapid cooling solidification and blooming.

Namely, in slabs produced by continuous casting, impurities (mainly sulfides) which act as fine dispersed precipitate for secondary recrystallization are irregularly distributed so that a concentrated segregation port commonly called black band appears near the central portion in the plate thickness direction and its grain structure contains many columnar grains elongated in the plate thickness direction.

The fine dispersed precipitate for the secondary recrystallization is formed by a fine precipitation of impurities contained in the steel caused by the control of cooling rate during the hot rolling or by the heat treatments after the hot rolling, but prior to this precipitation treatment, it is necessary to dissolve these impurities during the slab heating.

However, in case of the slabs produced by continuous casting, it is necessary to heat the slabs at higher temperatures if it is intended to dissolve also the concentrated segregation portion which exists at the center portion in the plate thickness direction. For this purpose, if the slab is heated on the higher side of the temperature range of l,260 l,400C, for example, as disclosed in Japanese Pat. No. 216,505, the grains grow too coarse and particularly, in the case of the slabs produced by continuous casting, development of the (ll0) 00l secondary recrystallization structure is remarkably hindered by the presence of the columnar grains. Therefore it is essential in the production of grain oriented electrical steel sheets by continuous casting that only the impurities which are dissolved in a slab heating temperature range which avoids excessive coarsening of the grains are used to form a fine dispersed precipitate effective for the secondary recrystallization.

The present invention has overcome the above difficulties in the production of grain oriented electrical steel sheet using solely MnS as the fine dispersed precipitate for the secondary recrystallization by continuous casting.

One of the objects of the present invention is to provide continuously cast slabs for grain oriented electrical steel sheets having excellent magnetic properties and to provide a method for producing the grain oriented electrical steel sheets from the continuously cast slab.

Another object of the present invention is to obtain grain oriented electrical steel sheets having excellent magnetic properties consistently using the continuously cast slabs.

Other objects of the present invention will be understood from the following descriptions and the attached drawings.

The features of the present invention lie in a continuously cast slab for grain oriented electrical steel sheets, comprising 0.025 0.060% of carbon, not less than 0.030% of manganese, not less than 0.010% of sulfur,

2.0 4.0% of silicon, not more than 0.005% of sol- .aluminum, with the balance being iron and unavoidable impurities, and the ratio of SiO /Al O being not more than 1.0, and also lie in a method for producing a grain oriented electrical steel sheet having excellent magnetic properties from the above continuously cast slab which comprises adding aluminum to molten steel containing controlled oxygen content not more than 0.12%, preferably not more than 0.10% in an amount of [oxygent% X 8 l kg/ton (molten steel) to deoxidize the steel and then continuously casting the molten steel.

According to the present invention, the magnetic flux density (B,,) in the rolling direction after the final annealing reaches more than l.83 wb/m which is equal or better than the magnetic properties obtained by the ordinary ingot-making method.

The present invention will be described in reference to the attached drawings.

FIGS. 1 and 2 are photographs showing continuously cast slabs. FIG. 3 is a graph showing the relation between the oxygen contents in molten steel for continuous casting before deoxidization and the magnetic flux density (8 of the final product. FIG. 4 is a graph showing the multiplier of Al use correlated to the oxygen contents in molten steel for continuous casting before deoxidization and the magnetic flux density of the final product, and FIG. 5 is a graph showing the relation between the amount of Al used and the ratio of Si- O /AI O In the present invention, the carbon content is limited to 0.025 to 0.060% for the following reasons. With a carbon content less than 0.025%, abnormality of the secondary recrystallization structure due to the columnar grains peculiar to the continuous casting material takes place frequently, thus causing adverse effects on the magnetic properties, while more than 0.060% of carbon requires a longer time for decarburization in the subsequent steps, thus causing economical disadvantages.

Regarding the silicon content, silicon is contained in a similer level as in an ordinary grain oriented electrical steel sheet, and less than 2% of silicon increases iron loss while more than 4% of silicon embrittles the steel, thus causing difficulties in the cold rolling. Thus the silicon content is limited to 2 4%.

Manganese is defined as not less than 0.030% and sulfur is defined as not less than 0.010%. When the manganese and sulfur contents are less than 0.03% and less than 0.01% respectively, the amount of MnS is not enough for forming the fine dispersed precipitate for the secondary recrystallization, and thus it is difficult to fully effect the secondary recrystallization.

While when manganese and sulfur are contained in too large amounts, dissolution of MnS which has precipitated in previous steps becomes difficult. Thus it is desirable that manganese is limited to not more than 0.075% and sulfur is limited to not more than 0.025%.

In a grain oriented electrical steel sheet, the improvement of the magnetization characteristics (B,,) can be attained by restricting the secondary recrystallization selectively grown from the primary recrystallization having the (110)[00l] orientation (so-called Goss orientation) at the time of final annealing and increasing the proportion of the grains having the (110)[00l] orientation.

For this purpose, the following considerations are important.

I. To homogenize the primary recrystallization structure before the secondary recrystallization.

2. To form a fine dispersed precipitate appropriate to the secondary recrystallization.

As contrast to the slabs produced by the ordinary ingot-making method, the slabs obtained by the continuous casting method have a peculiar interior nature and have inferior uniformity in the chemical composition and the grain structure so that the magnetic properties of the final products often are inferior. The materials produced by continuous casting are subjected to rapid cooling in an exceedingly flat mold having its width several times of its thickness so that columnar grains elongated from the slab surface in the thickness direction are predominant as shown in FIG. 1 and moreover the chemical components segregate in the thickness direction as shown in FIG. 2 so that a concentrated segregation portion commonly called as black band appears near the central portion in the thickness direction.

The columnar grains in the continuously cast slab as shown in FIG. I develop further in the slab heating step before the hot rolling, and this is more remarkable as the slab heating temperature reaches a high temperature more than l,350C for example.

These grown columnar grains cause abnormality in the primary recrystallization structure after the hot rolling and cold rolling, and reduce the proportion of the grains having the [001] orientation. Thus, in the abnormal structure, there exist elongated grains along the rolling direction so that the homogeneity of the primary recrystallization is hindered, and these elongated grains have a crystal orientation in which the ll0 axes are parallel and hinder growth of the grains having the (110) [001] orientation at the recrystallization step in which the grains having the (110) [001] orientation grow selectively. Also, as shown in:

FIG. 2, the segregation portion near the center in the thickness of the continuously cast slab makes it difficult to dissolve MnS in the subsequent slab reheating step, which MnS is used in the present invention as the fine dispersed precipitate for the secondary recrystallization, and also makes it very difficult to obtain uniform distribution of [Mn] and [S].

In this case, in order to obtain a uniform distribution of MnS as the fine dispersed precipitate in the secondary recrystallization, the slab heating temperature may be increased (for example more than l,350C),

but this causes an abnormal growth of the columnargrains and increases the abnormal structure in the primary recrystallization structure, and in this way the proportion of the grains having the (110) [001] orientation is lowered.

Therefore, in order to produce a grain oriented electrical steel sheet having excellent magnetic properties by continuous casting, it is necessary to select the slab heating temperature range to avoid the increase of abnormal structure in the primary recrystallization structure and to use MnS which has been dissolved in this temperature range to produce a precipitating dispersed state effective for the secondary recrystallization. MnS used as the fine dispersed precipitate for the secondary recrystallization is dissolved during the slab heating step and finely precipitates during the subsequent hot rolling or the heat treatment of the hot rolled sheet. It is particularly important to produce the MnS precipitate in a form effective for the secondary recrystallization in the hot rolling stage or the heat treating step of the hot rolled sheet, because it is very difficult to obtain uniform distribution of [Mn] and [S] due to unsatisfactory dissolution of MnS of the concentrated segregation portion in case of the material produced by continuous casting.

Based on the above facts, the present inventors have succeeded in obtaining a grain oriented electrical steel sheet having excellent magnetic properties by subjecting molten steel to a special deoxidization treatment with Al before continuous casting so as to convert the precipitation form of MnS into a form effective for the secondary recrystallization and in the present invention MnS is used solely as the fine dispersed precipitate for the secondary recrystallization.

It is known to produce a grain oriented electrical steel sheet by using only MnS as the fine dispersed precipitate for the secondary recrystallization without using AlN, and products obtained by this method have been widely accepted.

According to the principle of this known method, however, MnS is solely used as the fine dispersed precipitate so that it is not necessary to intentionally add Al which forms a nitride (AlN), and Al is not added as deoxidizer because if Al is used as deoxidizer it is nec essary to severely control the Al addition in order to control the amount of sol.Al after deoxidization by Al and Si in combination. The above concept has been predominantly accepted in the conventional arts.

According to the present invention which has been completed after many years of study on Al deoxidization, the oxygen content in the molten steel before deoxidization is restricted and Al is intentionally added to the molten steel for deoxidization in a range which is not enough for forming AlN and for it to form a combined dispersed precipitate with MnS.

In the present invention, it has been found that there is a strong relation between the oxygen content in the molten steel before deoxidization or the amount of A] used and the magnetization characteristics of the final product. FIG. 3 shows the relation between the oxygen content in the continuous casting molten steel containing about 3% Si before deoxidization and the magnetic flux density (B of the final products (0.30 mm and 0.35 mm thickness). In this case, the amount of Al used for the deoxidization is [oxygen(%) 8-15] kg /ton (molten steel). As clear from FIG. 3, a remarkable improvement of the magnetic flux density is observed when the oxygen content in the molten steel before deoxidization is not more than 0.12%, desirably not more than 0.10%.

Next, FIG. 4 shows the relation between the multiplier (a) for Al use correlated with the oxygen content (0.05 0.10%) in the continuous casting molten steel before deoxidization and the magnetic flux density (B of the final products (0.30 mm and 0.35 mm thickness).

It is understood from FIG. 4, that when the relation between the amount of Al used and the oxygen content in the molten steel is:

[ y'g X a kg/ton (molten steel) is maintained at a value not less than 8 at the time of deoxidization, a high level of magnetic flux density can be obtained stably.

Meanwhile when the amount of Sol.Al exceeds 0.005% it forms AlN which acts on the secondary crystallization as a combined fine dispersed precipitate with MnS, and the secondary crystallization is greatly influenced even by a small variation in the amount of sol.Al, and thus severe control is required for stabilization of the various properties in the commercial production. Although it is observed that excellent properties can be obtained with a low slab heating temperature by using MnS and MN as a combined fine dispersed precipitate, when the amount of sol.Al is low, for example, in a range between 0.006 and 0.009%, the secondary recrystallization grains grow coarse when the strict combination of the slab heating temperature and other treating conditions is misselected, so that the steel surface will have an uneven pattern corresponding to the secondary recrystallization, or the magnetic characteristics tend to be lower due to unsatisfactory secondary recrystallization.

Thus, in the present invention, Al is used in an amount not less than [oxygen(%) X 8]kg/ton, and its upper limit is limited to such .an amount that sol.Al is contained less than 0.005%. For this purpose, the upper limit of Al to be used is about [oxygen(%) X 15 ]kg/ton. Within the above range, the most preferable range is one which corresponds to the so|.Al range of 0.002 to 0.005%. When the soLAl is less than 0.002%, homogeneity of the primary recrystallization and uniformity of the fine dispersed precipitate become hard to be attained and unsatisfactory secondary recrystallization easily takes place when the continuous coating conditions or treating conditions in the subsequent step vary.

The present inventors investigated the relation between the amount of Al used and impurities contained in steel materials produced by continuous casting in order to determine the effects of Al deoxidization on improvement of the magnetic flux density. The results are shown in FIG. 5. It is understood from the results that as the amount of Al used increases, the ratio of Si- O /AI O tends to decrease, and that desired results are obtained in the present invention when the ratio of Si- O /Al O is not more than 1.1. Thus it is also understood from the results shown in FIG. 5 that the ratio of SiO /AI O and therefore the precipitation form of MnS have a large influence on the magnetic properties (particularly the magnetization characteristics which is are related to the secondary recrystallization). Thus, the deoxidization method of the present invention enhances the effect of the-MnS which is used as the fine dispersed precipitate for the secondary recrystallization. It has thereby become possible to produce a grain oriented electrical steel sheet having excellent magnetic properties by continuous casting.

In practicing of the present invention, it is natural to adopt a slab heating temperature (for example l,320C) which can avoid remarkable coarsening of the columnar grains in the slab which causes increase of the abnormal structure in the primary recrystallization structure, and it is also natural to control fully the amounts of [Mn] and [S] in order to approach a complete solid solution of MnS.

For this purpose, in the examples shown in FIG. 3 and FIG. 4, a composition of 0.050 0.065% Mn, 0.015 0.050% S is used to obtained very excellent results.

The present invention will be fully understood from the following example.

During tapping molten steel from a 100 ton converter. Al was placed on the bottom of a ladle, and Si was added to the molten steel stream from the converter to effect deoxidization and the composition was adjusted. This molten steel was continuously cast to obtain slabs of 200 mm X 1,030 mm cross section.

The oxygen content in the molten steel before deoxidization. the amount of Al used the multiplier. and the slab composition are shown in Table 1.

The slabs A and E produced according to the present invention, the slab B has an oxygen content before deoxidation outside the range of the present invention, the slab C has an aluminum content outside the range of the present invention, the slab D is one obtained by a conventional method using almost no Al, and the slab F has a carbon content outside the range of the present invention.

As clear from Table 2, the materials A and E of the present invention give magnetic properties with higher stability as compared with the materials B, C, D and F.

Table l Slab Compositionfll Oxygen Use of Al Content SiO. before C Si Mn S SoLAl Deoxidi- Amount Multi- A1 zationr, kg/ton plier A 0.040 3.15 0.056 0.018 0.003 0.89 0.08 0.75 9.4 B 0.039 3.15 0.055 0.017 0.002 1.12 0.1-1- l.27 8.9 C 0.041 3.17 0.053 0.018 0.001 1.27 0.08 0.45 5.6 D 0.033 3.19 0.062 0.022 trace 3.10 0.09 0 0 E 0.034 3.16 0.061 0.019 0.005 0.70 0.10 1.25 12.5 F 0.023 3.12 0.059 0.020 0.001 1.02 0.1l 1.00 9.0

The slabs A, B, C. D. E and F were heated at 1.300C for 3 hours. hot rolled to a thickness of 2.3 mm and then cold rolled to a thickness of 0.30 mm by an ordinary two-pass cold rolling method to obtain the final products.

The magnetic properties in the rolling direction of these products are shown in Table 2.

What is claimed is:

l. A continuously cast slab for producing grain oriented electrical steel sheets consisting essentially of 0.025 to 0.060 carbon, not less than 0.030% manganese, not less than 0.010% sulfur, 2.0 to 4.0% silicon, not more than 0.12% oxygen, not more than 0.005% sol. aluminum, said slab having been deoxidized by adding aluminum in an amount of not less than (oxygen 8-15) kg/ton, with the balance being iron and unavoidable impurities and the ratio of SiO /Al- O being not more than 1.1.

2. A continuously cast slab according to claim 1, in,

which the sol.aluminum is 0.002 0.005%. 

1. A CONTINUOUSLY CAST SLAB FOR PRODUCING GRAIN ORIENTED ELECTRICAL STEEL SHEETS CONSISTING ESSENTIALLY OF 0.025 TO 0.060 CARBON, NOT LESS THAN 0.030% MANGANESE, NOT LESS THAN 0.010% SULFUR, 2.0 TO 4.0% SILICON, NOT MORE THAN 0.12% OXYGEN, NOT
 2. A continuously cast slab according to claim 1, in which the sol.aluminum is 0.002 - 0.005%. 